The invention relates to a method for visualizing a spatially resolved data set and to a use of this method for generating three-dimensional representations of an object.
Visualizing a spatially resolved data set through pictorial representations corresponds to a constantly increasing need in many technical and industrial areas, but also in modern medical diagnostics and therapy. Numerous image generating examination methods such as e.g. computer tomography, nuclear spin tomography or imaging by means of ultrasound, in which representations of organs or regions of the human body are produced on the basis of data sets resulting from measurements, are being used in modern medicine with great success.
Trans-esophageal heart imaging by means of ultrasound, which is chiefly used for diagnostic purposes, may be mentioned here as an example of an application. Many of the ultrasound systems which are usual nowadays however produce only two-dimensional images, and it is often very difficult even for specialists to analyze the three-dimensional anatomy on the basis of such two-dimensional images. Therefore much effort is being invested in representing the anatomy by means of three-dimensional ultrasound pictures.
Ultrasound systems of this kind which produce three-dimensional images are also already known from the prior art. For this the anatomy to be imaged, e.g. a ventricle of the heart, is first sampled region-wise by means of ultrasound and then a three-dimensional image is reconstructed in a data processing system from the echo signals obtained in this manner.
A substantial disadvantage of these known three-dimensional ultrasound systems consists in that the time which is required for the generation of a three-dimensional image with sufficient spatial resolution is relatively long. In known three-dimensional systems one is still far away from the video frequencies and their image build-up times of typically {fraction (1/25)} of a second so that no moving representations can be achieved. For applications such as for example the navigation or the localizing of instruments which are required within the body, e.g. heart catheters for the ablation of stimulus lines in the heart or other low-invasion instruments, such long times for the image generation are unsatisfactory because they are opposed to the requirement of a rapid and precise localization of the momentary position of the instruments in the body. This holds in particular for those cases in which the examined or treated part of the body moves, for example in operations on or examinations of the beating heart.
In the use of very modern sampling systems the relatively long time which is required for the generation of a three-dimensional image is caused less by the data acquisition per se, but rather by the visualization of the data set, which means the,generation of a pictorial representation from the spatially resolved data set. Very rapidly sampling ultrasound probes have namely already been developed, by means of which the volume of interest is simultaneously sampled in a plurality of planes. Ultrasound probes of this kind comprise for example a plurality of pivotal ultrasound transducers which are arranged linearly or more general in an array, of which a plurality are operated in parallel so that they enable a simultaneous sampling of a plurality of planes. Through this the data sets for the imaging are very rapidly provided.
The methods and algorithms which are used nowadays for the three-dimensional representation of spatially resolved data sets are however very computation-intensive and lead to generation times for an individual three-dimensional image which lie in the range of seconds even when very rapid and high performance computers are used. Known methods of this kind are thus not suitable for real time applications.
Starting from this prior art it is thus an object of the invention to provide a particularly rapid method for visualizing a spatially resolved data set. The method should especially enable the generation of a three-dimensional representation of an object from a spatially resolved data set which represents the volume-resolved sampling of the object in significantly less time.
Thus in accordance with the invention a method for visualizing a spatially resolved data set is proposed, the data of which are in each case associated with a volume element, the position of which is described by coordinates in a non-Cartesian measurement coordinate system, in which method the data are loaded into a graphics engine as texture maps and then a pictorial representation is generated through superposition of texture maps.
Through these measures the method in accordance with the invention becomes enormously rapid since for each texture map to be represented it need now be transmitted to the graphics engine (also known as graphics accelerator) only at which coordinates of an output unit, for example of a monitor, the corner points of the texture map come to lie. The graphics engine then represents the corresponding texture map perspectively correctly between these corner points. This brings about a drastic reduction in the computational effort. Through superposition of the individual texture maps to be represented a pictorial representation of the data set can thus be generated in a very short time. In addition it is not necessary to transform the data set to be represented into a Cartesian coordinate system, i.e. to carry out a xe2x80x9cresamplingxe2x80x9d of the data set, before the data are loaded into the graphics engine as texture maps. This means a further reduction in the computational effort and thereby an additional gaining in time. Through the omission of a transformation into Cartesian coordinates, furthermore, the three-dimensional oversampling is avoided, which leads to a multiplication of the data in particular in a transformation from curvilinear coordinates, such as cylindrical or spherical coordinates, into Cartesian coordinates, and thus to a considerable increase in the computational effort and the memory and time requirement.
Through the intentional use of the graphics engine the method in accordance with the invention permits a significantly more rapid build-up of the pictorial representation than previously known methods for visualizing spatially resolved data sets. This is advantageous in particular for applications in which the data sets are current resolved measurement values or, respectively, are based on such and these measurement values must be transformed into a three-dimensional representation in as short a time as possible. The method in accordance with the invention namely enables a visualization of these measurement values and thus a visualization of three-dimensional structures in real time. Thus for example a continuous and always current three-dimensional view of a beating heart can be realized in that the volume-resolved measurement signals which are picked up by an ultrasound probe are imaged on a monitor by means of the method in accordance with the invention as three-dimensional representations. As a result of the graphics engine the method in accordance with the invention is so rapid that picture rates of several tens of images, for example twenty images, per second can be realized.
A further advantage of the method in accordance with the invention lies in that it is very economical, since it can be carried out with graphics engines available on the market and without further apparative cost and complexity.
A further advantage of the method in accordance with the invention is that it is not bound to a special coordinate system in which the data set must be present and is thus very flexible. The method is suitable for all locally orthogonal coordinate systems, in which spatially resolved data sets are normally present. Therefore the data are preferably loaded into the graphics engine in the original measurement coordinate system, which means without being transformed to another coordinate system, since this saves computational effort and time and is less subject to errors, since no resampling is required.
If the data are for example measurement values which result from a volume-resolved sampling of an object, the measurement coordinate system in which the measurement values are present is normally predetermined by the sampling apparatus used or by its method of operation respectively. In ultrasound probes, e.g. the measurement coordinate system is typically a cylindrical coordinate system. With the method in accordance with the invention, however, data sets which are present in other, in particular in locally orthogonal, coordinate systems can also be represented without it being necessary to transform the data into a Cartesian system beforehand. The method is thus very flexible and is suitable for a large number of sampling systems, with the data in each case preferably being loaded as texture maps into the graphics engine in that measurement coordinate system in which they were measured. This measurement coordinate system is usually predetermined by the respective sampling system.
The texture maps are preferably adapted to the measurement coordinate system in such a manner that in each case one of the coordinates of the measurement coordinate system has a constant value within a texture map. Then each texture map corresponds to a surface represented by the data set on which one of the coordinates of the measurement coordinate system has a constant value. Furthermore, it is advantageous if in each case a set of texture maps is set up in the graphics engine for each coordinate of the measurement coordinate system, with in each case the same coordinate of the measurement coordinate system having a constant value within the texture maps which belong to the same set. For example a plurality of x1 texture maps are set up for an x1 coordinate of the measurement coordinate system. The data within an x1 texture map in each case all belong to the same value for the x1 coordinate of the measurement coordinate system, whereas the data which belong to two different x1 texture maps differ in the value of the x1 coordinate to which the data belong. The entire set of x1 texture maps then represents a plurality of surfaces which are topologically ordered with respect to this coordinate x1 and on which in each case the coordinate x1 has a constant value. Through this measure the texture maps are matched to the symmetry of the measurement coordinate system, through which the generation of the pictorial representation becomes simpler and requires a lesser computational effort.
All sets of texture maps are preferably used for the generation of the pictorial representation; for example the texture maps of all sets are projected simultaneously onto the output unit, which means that they are represented in the same image, where their intensity contributions are summed.
Preferably, the surfaces of the pictorial representation which are generated by means of a texture map are in each case with respect to their intensity weighted with a factor in which the orientation of the surface relative to the direction of view is reflected. Through a trigonometric weighting of this kind, undesirable stripe patterns which arise through the distances between the texture maps can be significantly reduced in the pictorial representation.
A further advantageous measure consists in subdividing texture maps which correspond to a curved or curvilinearly bounded surface into sub texture maps. This means that those coordinates of the measurement coordinate system which describe curves or curvatures, for example angular coordinates, are locally linearized. The surfaces which are perpendicular to the direction which is described by the locally linearized coordinate are curved surfaces. Through the local linearization, such surfaces are approximately represented by a plurality of planar n-sided polygons, e.g. quadrilaterals.
In order to generate as realistic a depth impression as possible preferably in the representation in the case of semi-transparent representations the individual texture maps are provided with a depth attenuation when represented,
For the further reduction of the formation of stripes in the generated representation it is furthermore advantageous when a closure texture map is added as an enyelope at the edges of texture maps which belong to the same set of texture maps and which edges in each case form an edge line, which closure texture map connects these edges and effects a modulation in the representation between adjacent edge lines. Through this measure it is taken into account that, depending on the direction of view or perspective respectively, the boundaries or edges respectively of individual texture maps no longer lie one over the other. This leads to intensity jumps in the representation because a different number of texture maps is visible in different image regions, or, expressed differently, the viewer looks into different image regions through a different number of texture maps. The stripe patterns caused by this at the transitions between different numbers of texture maps can be effectively prevented through the measure of the modulated closures, which means the addition of the closure texture maps.
The closure texture map is preferably perpendicular to the edges of the set of texture maps bordering on it and contains the edge lines of the texture maps bordering on it and effects a linear interpolation of the brightness between adjacent edge lines. This can be realized in particular by the closure texture map being generated in that the edge lines of the bordering texture map are taken over identically into the closure texture map and a line of brightness zero is inserted ahead of each of these edge lines. This is a particularly simple and efficient method of generating the closure texture maps because it implies no additional computational effort. The function of the linear interpolation, also designated as linear shading, of which every graphics engine is capable, is then used to realize the modulated closures without additional effort.
The method in accordance with the invention is suitable in particular for data sets in which the measurement coordinate system is a cylindrical or spherical coordinate system. These coordinate systems represent important applications because many of the sampling apparatuses which are usual nowadays operate in these coordinate systems.
In a preferred use of the method the data set is based on measurement values which represent the volume-resolved sampling of an object, and a three-dimensional, in particular semi-transparent, representation of the object is generated as a pictorial representation. As a result of the enormous speed of the graphics engine, namely, the method in accordance with the invention enables the visualization of data sets of this kind in real time, with it being. in principle irrelevant by means of which sampling method the data set was generated. The data set can for example be based on ultrasound measurement values which are representative for the three-dimensional structure of an object.
In a preferred embodiment the method in accordance with the invention is used for the rapid generation of three-dimensional representations of an object, in particular of a human body or parts thereof on the basis of sampling data obtained by measurement. The method in accordance with the invention is especially suitable for medical purposes. Through its rapidity with which a three-dimensional real time representation of spatially resolved data sets can be realized, the method in accordance with the invention enables for example a continuous, always current three-dimensional representation of organs of the human body, e.g. of the beating heart. This is a considerable advance for therapeutic or diagnostic procedures in which instruments such as for example catheters must be localized and navigated in the interior of a human or animal body or organ respectively.
But the method in accordance with the invention can also be advantageously used in other technical fields as well, e.g. in the representation of radar measurement data, in remote sampling or in material testing.
The invention will be described in the following with reference to the drawings and with reference to exemplary embodiments.